1. Field of the Invention
This invention relates to a method and an apparatus for controlling an image demonstrated on a liquid crystal display surface based on an input image signal or a photographed image signal, and to an imaging method and apparatus. More particularly, it relates to a method and apparatus for controlling an image, which may be convenient for adjusting the contrast, and to an imaging method and apparatus.
2. Description of Related Art
In an imaging apparatus, typified e.g. by a video camera, an image for photographing is displayed on a screen of an electronic viewfinder formed by light modulating elements, such as liquid crystal display devices. On the back surface of this electronic viewfinder, there is mounted a backlight formed e.g. by a planar phosphor tube. This backlight is caused to emit light to illuminate the liquid crystal display device from the back side to improve the viewing characteristics.
For causing the backlight to emit light, horizontal synchronization signals are first separated from the input image signal. Responsive to the separated horizontal synchronization signals, pulse signals are generated and supplied to a backlight driving circuit. FIG. 1 shows pulse signals generated in succession at a period of the horizontal frequency fH. By providing the horizontal blanking period, it becomes possible to synchronize the period of light emission of the backlight with the image display state to prevent flickering of the displayed image.
Moreover, by changing the illumination brightness of the backlight, it becomes possible to adjust the contrast of the image displayed on the electronic viewfinder. That is, by changing the pulse width PW of the pulse signal of FIG. 1, supplied to a driving circuit of the backlight, it is possible to control the average current value IE indicative of the illumination brightness. For decreasing or increasing the illumination brightness of the backlight, the pulse width PW may be narrowed or widened, respectively. Such control of the average current value IE, implemented by changing the pulse width PW in this manner, has so far been implemented using a pulse width modulation circuit (PWM circuit). With the PWM circuit, the ON/OFF period of the pulse signal is controlled to change the pulse width PW.
Meanwhile, if, when the signal level of the input image signal is extremely small, the pulse signals are turned on and off using the PWM circuit as described above to effect the backlight switching, this switching operates as a noise, thus severely disturbing the displayed image.
With this in view, a dimmer system 140 for controlling the light emitting brightness of a backlight 130 by controlling the amplitude of the boosting AC voltage, as shown in FIG. 2, has been proposed.
This dimmer system 140 is made up by a DC power supply Ba, a dimmer control IC circuit 141, a dimmer switching transistor 142, an inductor 143 and a Royer oscillator 150.
The dimmer control IC circuit 141 includes an input terminal 141a, by which it receives a control signal controlling the brightness of the backlight 130 to output pulses in dependence on the control signal level. The dimmer switching transistor 142 has its duty controlled based on pulses from the dimmer control IC circuit 141 to make intermittent current supply from the DC power supply Ba to the inductor 143. The inductor 143 sends the DC voltage, intermittently supplied by the dimmer switching transistor 142, to the Royer oscillator 150. This Royer oscillator 150 generates the boosting AC voltage based on the DC voltage supplied from the inductor 143. This boosting AC voltage is supplied via a capacitor, not shown, to the backlight 130 to cause the current to flow through the phosphor tube. This enables the dimmer system 140 to dim and turn on the backlight 130.
That is, with this dimmer system 140, the amplitude of the boosting AC voltage may be changed by the value of the DC voltage supplied from the DC power supply Ba to allow control of the average current value IE.
However, with the above-described conventional dimmer system 140, the smallest value of the average current value IE is limited from the perspective of maintaining a stable discharge current of the backlight. Consequently, since the entire image becomes lighter, there is raised a problem that, especially if an image signal of a black level is input, the black color is displayed in a floating fashion, while the range of possible variation of contrast is reduced. In particular, since the illumination brightness of the conventional backlight is controlled independently of the luminance signal level of the displayed image, the contrast of an actually demonstrated image cannot be flexibly adjusted in relation to the input image signal.